Pitfall trap. Rather than having a standard pit where a character falls and suffers a little bit of damage you can make a whole room into the trap. The room shown below is 30 feet by 50 feet with two doors. The first in in the 30 foot wide wall and has two hooks mounted into the wall on either side (Door A). The second door hangs in the middle of the 50 foot wall and is a drop straight down (Door B).
If the characters enter the room through the door A, they have avoided the trap. However if they enter the room through door B the characters are going to have a problem. For this trap to be very effective door B must be stuck closed where the characters would have to put some muscle into getting through (Strength checks). If more than one character is trying to open the door, the first three characters are affected by the trap. The first character must make a Dexterity check with disadvantage. If the check fails they fall 10 feet onto the ramp (1d10 damage) and then they roll down the into the 10 foot deep pit (another 1d10 damage) and the end of the room. The second character helping with the door must make a Dexterity check or fall and if there is third character they too must make a check with advantage or fall in the room.
If your characters are higher levels you could make the following adjustments to make the trap even more dangerous. If the first character succeeds but the second character fails, the first character must make a second Dexterity check with disadvantage or fall in along with the second character. If the first and second characters succeed but the third character fails then the first two characters will need too make additional Dexterity checks with disadvantage. If either (or both) characters fail this check they fall into the room along with the third character.
Getting out of the pit will provide to be even more difficult. Any characters who have not fallen into the room will need to find away to door A (flight, teleport or running through the halls until they find their way to the other door) and set up at least 60 feet of rope to help the characters get out. Using the rope ensures a successful rescue (no additional ability checks). Trying to climb out without rope means additional Dexterity checks every 10 feet with failure causing the character to roll down the slope again and then fall into the pit (another 1d10 of damage).
This trap could seriously damage (or kill) a party with each falling character taking at least 2d10 points of damage right away. Make sure this trap doesn’t kill your evening as easily as it could kill your party.